The present invention relates to a variable air volume system and, more particularly, to a variable air volume system having a plurality of zones wherein the thermal transfer rate with respect to each of such zones is controlled for improved efficiency and environmental comfort.
Heating, ventilating and air-conditioning (HVAC) systems are used to both heat and cool the air within an enclosure, e.g., a building or zone within such building. An HVAC system typically includes a heating unit, a cooling unit, a supply air fan, a supply duct for directing air into the enclosure, and a return duct for removing air from the enclosure. It will be appreciated by those skilled in the art that HVAC systems are generally designed to operate in one of three modes: a heating mode to heat the enclosure, a cooling mode to cool the enclosure and a economizer mode to ventilate the enclosure, as well as cool the enclosure under certain conditions. The economizer mode typically utilizes an outdoor air damper, commonly referred to as an economizer, that can be selectively opened to allow the return air to mix with fresh outside air.
As will be recognized by those skilled in the art, there is typically a control system associated with an HVAC system, such control system including a thermostat (typically located within the enclosure) and associated hardware/software for controlling the components of the particular HVAC system in response to preprogrammed instructions. Typically, the control system allows a user to pre-select one of the three operating modes, as well as selecting a desired temperature for the enclosure. Thereafter, the control system activates either the heating or cooling portion of the HVAC system to maintain the pre-selected temperature within the enclosure. Under certain conditions the economizer mode may be able to maintain the enclosure at the pre-selected temperature.
One common HVAC system is referred to as a variable air volume (VAV) system. A VAV system utilizes individual flow control boxes which control the air flow from a main supply duct into an individual zone of a building, e.g., an office, conference room, etc. Particularly, the individual flow control boxes regulate the volume of air flow entering the zone between a minimum flow volume and a maximum flow volume, generally by moving a damper or valve in the flow control box. The damper is moved in response to changes in the temperature in the room as measured by a thermostat in such room. The measured room temperature is compared to a room set point temperature, and the air flow entering the room (whether cold air for cooling or hot air for heating) is regulated accordingly.
Many VAV systems are designed to operate with a fixed supply air temperature (e.g., 55° F. in cooling mode). Other VAV systems are designed to regularly reset the supply air temperature (e.g., 55° F.-60° F. in cooling mode) in response to the thermal load. In either system, the supply air temperature can undergo a significant temperature change over a very short period of time. Particularly, a VAV system utilizing an on/off heating or cooling unit will experience a significant temperature swing each time the unit is cycled on or off. For example, if an additional stage of a direct expansion (DX) cooling unit is turned on, there will be a sudden decrease in the temperature of the supply air (e.g., 5°-7° F.). Likewise, turning off a stage of a DX cooling system will result in a sudden increase in the temperature of the supply air (e.g., 5°-7° F.). Conventional systems continuously cycle the heating or cooling units to maintain the temperature of the supply air at the selected point.
Those skilled in the art will appreciate that changes in the temperature of the supply air in a variable air volume system often result in uncomfortable temperature swings within the individual zones. Ideally, the flow control box maintains the room temperature of the zone at the desired set point by opening and closing the damper, thus regulating the volume of air entering the zone. If, for example, a VAV box is allowing approximately 1,000 ft3/min of cold air to enter the zone to maintain the temperature of the zone at the desired set point (or within the designed temperature range), it will be appreciated that a decrease in the temperature of the supply air (assuming the system is in a cooling mode) will result in the overcooling of the zone.
Specifically, the flow control box will continue to allow the same amount of air (e.g., 1,000 ft3/min) to enter the zone, but because the supply air is at a decreased temperature, the temperature in the zone will decrease. This decrease in temperature will likely bring the temperature of the zone outside of the designed temperature range, and into an uncomfortable zone for the occupants. Due to the inherent time delays associated with all HVAC systems, the room will have already reached the undesirable temperature before the system can signal the flow control box to decrease the flow of air into the zone. Stated differently, the flow control box will eventually decrease the flow of air into the zone based on the room temperature falling below the set point temperature, but this will happen in effect “after the fact.”
A similar event will occur if the supply air temperature suddenly rises (due to a stage of cooling being turned off) in which case the temperature in the zone may rise to an uncomfortable level before the system signals the flow control box to increase the flow of air into the zone. Of course, these same undesirable temperature swings are experienced when the system is in a heating mode or when the supply air temperature is reset, either automatically or by a system operator.
As mentioned, certain prior art VAV systems are designed to reset the supply air temperature. These systems, although having the capability to reset the supply air temperature over a limited range by, for example, measuring the temperature of the return air, do not actually match the temperature of the supply air to meet the thermal load on the system. For example, the system may only need supply air at 65° F. to satisfy the total cooling load, but will nonetheless continue supplying air at 60° F. (or lower) in accordance with the system's specifications. Such systems are therefore unable to realize this potential savings in energy costs. Likewise, the prior art VAV systems may overheat the supply air when the system is in a heating mode.
In addition to this mentioned inefficiency in prior art VAV systems, overcooling of the supply air often results in environmental discomfort to the occupants of the building. Because the supply air is colder than necessary, the flow control boxes will need to restrict the flow of air into the various zones. This decrease in air flow can result in a problem referred to as “dumping”, which results when the exit velocity of the supply air into the zone is too low to adequately mix the cold supply air with the warmer room air thus causing the cold supply air to simply “dump” into the zone and onto the occupants. Moreover, the restricted air flow into the zones also reduces the indoor air quality (IAQ) in such zones.
Finally, the flow control boxes of prior art VAV systems are unable to provide an indication of an existing unmet cooling/heating load in a particular zone(s). For example, a prior art flow control box can provide an output signal indicating that the box is providing maximum flow volume into the zone. However, this prior art output signal does not indicate whether this maximum flow volume is satisfying the thermal load in the zone or whether additional cooling/heating is still required. Typically, additional cooling/heating in a VAV system is provided by resetting the temperature of the supply air. In practice, this unmet cooling/heating load in a prior art VAV system will only be discovered through occupant complaints that the zone is either too hot or too cold.
There is therefore a need in the art for a method of controlling a variable air volume system, as well as a controller, which anticipates and limits/prevents the undesirable temperature swings in the various zones of a building which result from the changes in temperature of the supply air due to system resetting and/or to cycling of the heating/cooling unit. There is a further need in the art for a VAV system which can provide a signal for the resetting of the supply air temperature in response to the thermal load on the building thereby realizing savings in energy costs, improving environmental comfort and improving indoor air quality. Finally, there is a need in the art for a VAV system which can provide an indication of an existing unmet cooling/heating load in a particular zone of the building.